The conventional development of low grade nickel sulphide serpentinitic ore bodies such as Mt Keith, Western Australia, have been limited by the requirement to produce a nickel concentrate product containing low levels of Magnesia (MgO) minerals. This constraint is usually referred to as the Iron to Magnesia ratio of the concentrate (Fe:MgO ratio). The MgO constraint is a result of a physical limitation of the downstream smelting process. The concentrate smelting process typically require Fe:MgO ratios of about 5.5, which corresponds to an MgO content of <5%. One means of increasing the Fe:MgO ratio is the addition of Fe to the concentrate however this is not an ideal solution as it dilutes the nickel content and reduces the smelter capacity. Hence the objective is to achieve an Fe:MgO ratio of 5.5 by rejecting MgO.
The conventional processing route for these types of ores involves crushing and grinding followed by concentration of the nickel through multiple stages of flotation. Conditions in the flotation process are optimised to recover Ni minerals and reject MgO minerals. This technology has been applied at Mt Keith and the concentrate Fe:MgO has averaged around 2.9 that corresponds to a MgO content of about 10%. Although not meeting the acceptable Fe:MgO specification, it has been possible for the smelter to accept this concentrate through blending the Mt Keith concentrate with concentrates from other locations. This blending has provided the smelter with the required 5.5 Fe:MgO ratio. However, the smelting requirement of the Fe:MgO of 5.5 limits the future amount of Mt Keith concentrate that can be smelted without blending with other concentrates.